This invention relates to devices for raising and lowering loads; particularly devices used by skyscraper workers, construction workers, and rescue personnel to raise and lower equipment or people.
A typical device uses a rope with one end connected to the load and the other end connected to a man, who exerts the force necessary to raise and lower the load. As the rope is pulled by the man, the load is raised. Release of the rope by the man causes the load to be lowered. If the man merely holds onto the rope, then the load is neither raised nor lowered, but is held stationary at a constant height. As a safety feature, a typical device has a brake that restrains the rope in one direction. One type of brake uses a stationary clamp to hold the rope between the clamp and a pulley. The clamp has teeth and thus prevents the rope from moving in one direction.
A typical braking system is activated in two ways. First, the braking system is periodically activated as the rope is pulled by the man. In this way the man can stop pulling at selected points and the rope will be locked in one direction. Second, the braking system is activated if the load is lowered at a rate above a selected threshold. In this way, if the man releases the rope at a point where the brake is not engaged, then the braking system will stop the load from falling once the load has reached a selected speed.
A typical raising and lowering device uses several pulleys to reduce the amount of force required to raise the load. At least one of the additional pulleys is a one-way pulley which rotates in one direction only. By using a one-way pulley, the advantages of a block and tackle arrangement are achieved when the load is raised, and when the load is lowered, a frictional force is produced because the one-way pulley cannot turn. Therefore, the rope slides over the pulley's surface creating friction. This allows the load to be lowered at a steady rate.